Ubuntu 17.10 shows my gigabit ethernet connection at 100Mbps

I have just installed Ubuntu 17.10 on my Toshiba Satellite L50t-a-140 laptop which has a gigabit ethernet wired adapter connected to a gigabit ethernet switch, but Ubuntu reports the link speed at 100Mbps. How can I work out why it is not reporting a gigabit ethernet connection?

~$ sudo lshw -C network
  *-network                 
       description: Ethernet interface
       product: AR8161 Gigabit Ethernet
       vendor: Qualcomm Atheros
       physical id: 0
       bus info: pci@0000:01:00.0
       logical name: enp1s0
       version: 10
       serial: 00:8c:fa:94:4f:09
       size: 100Mbit/s
       capacity: 1Gbit/s
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi msix bus_master cap_list ethernet physical tp 10bt 10bt-fd 100bt 100bt-fd 1000bt-fd autonegotiation
       configuration: autonegotiation=on broadcast=yes driver=alx duplex=full ip=192.168.1.4 latency=0 link=yes multicast=yes port=twisted pair speed=100Mbit/s
       resources: irq:16 memory:b0500000-b053ffff ioport:3000(size=128)
  *-network DISABLED
       description: Wireless interface
       product: QCA9565 / AR9565 Wireless Network Adapter
       vendor: Qualcomm Atheros
       physical id: 0
       bus info: pci@0000:02:00.0
       logical name: wlp2s0
       version: 01
       serial: 28:e3:47:02:df:c2
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress bus_master cap_list rom ethernet physical wireless
       configuration: broadcast=yes driver=ath9k driverversion=4.13.0-38-generic firmware=N/A latency=0 link=no multicast=yes wireless=IEEE 802.11
       resources: irq:17 memory:b0400000-b047ffff memory:b0480000-b048ffff